In the recovery of hydrocarbons, such as oil and gas, from natural hydrocarbon reservoirs, extensive use is made of wellbore treatment fluids such as drilling fluids, completion fluids, work over fluids, packer fluids, fracturing fluids, conformance or permeability control fluids and the like.
In many cases significant components of wellbore fluids are thickening agents, usually based on polymers or viscoelastic surfactants, which serve to control the viscosity of the fluids. Typical viscoelastic surfactants are N-erucyl-N, N-bis(2-hydroxyethyl)-N-methyl ammonium chloride and potassium oleate, solutions of which form gels when mixed with corresponding activators such as sodium salicylate and potassium chloride.
Conventional surfactant molecules are characterized by having one long hydrocarbon chain per surfactant headgroup. In the viscoelastic gelled state these molecules aggregate into worm-like micelles. Gel breakdown occurs rapidly when the fluid contacts hydrocarbons which cause the micelles to change structure or disband.
In practical terms the surfactants act as reversible thickening agents so that, on placement in subterranean reservoir formations, the viscosity of a wellbore fluid containing such a surfactant varies significantly between water- or hydrocarbon-bearing zones of the formations. In this way the fluid is able preferentially to penetrate hydrocarbon-bearing zones.
N-erucyl-N, N-bis(2-hydroxyethyl)-N-methyl ammonium chloride is a waxy solid at room temperature that is difficult to dissolve in water. It is conventional to deliver it to the wellhead in the form of a concentrated solution in isopropyl alcohol. This is then mixed with water (and other additives) to form the treatment fluid.
Potassium oleate, on the other hand, is usually formed in situ by the on-line mixing oleic acid and potassium hydroxide solutions. However, the transportation and subsequent rig-site pumping of concentrated potassium (or sodium) hydroxide solution is potentially hazardous and the exothermic acid-base reaction may result in excessive heating.
The application of viscoelastic surfactants in both non-foamed and foamed fluids used for fracturing subterranean formations has been described in several patent applications and patents, e.g. EP-A-0835983, U.S. Pat. No. 5,258,137, U.S. Pat. No. 5,551,516, U.S. Pat. No. 5,964,295 and U.S. Pat. No. 5,979,557.
The use of viscoelastic surfactants for water shut off treatments and for selective acidizing is discussed in GB-A-2332224 and Chang F. F., Love T., Affeld C. J., Blevins J. B., Thomas R. L. and Fu D. K., “Case study of a novel acid diversion technique in carbonate reservoirs”, Society of Petroleum Engineers, 56529, (1999).
U.S. Pat. No. 5,981,446 discloses a method of producing a fracturing fluid by the addition of a dry mix blend to the mixed water being pumped into a well. However, the viscoelasticity of the fracturing fluid was generated by the dissolution and subsequent crosslinking of a water-soluble polysaccharide, and not by surfactant molecules. Also, ensuring adequate hydration of the polymer and providing sufficient energy to mix the blend into the mixed water is problematic for polymer-forming blends.